This invention relates generally to lasers, and, more particularly to unstable optical resonators for use within a laser system in which the resonator has a self-imaging aperture.
The operation of a laser is based upon the fact that the atomic systems represented by the atoms of the laser medium can exist in any of a series of discrete energy levels or states, the systems absorbing energy in the optical frequency range in going to a higher state and omitting it when going to a lower state. The laser medium may be a solid, liquid or gas. In the case of a solid wherein a ruby is used as a laser material, three energy levels are utilized. The atomic systems are raised from the lower or ground level to the higher of the three levels by irradiation from, for example, a strong light source which need not be coherent but should preferably have a high concentration of energy in the absorbing wavelengths. A radiationless transition then occurs from the highest state to an intermediate or metastable state. This is followed by a transition with photo emission from the intermediate state back to the ground state. It is the last transition that is of interest since this transition is the source of the coherent light or electromagnetic energy produced by the laser.
The operation of raising the energy level of the laser material to produce the desired photo emission is referred to in the art as "pumping" and when more atoms reach an excited metastable state than remain in a lower energy level a "population inversion" is said to exist. The active medium in the laser is made optically resonant by placing reflectors or other optical devices, hereinafter referred to as the resonator of the laser, at the end thereof, forming the resonant chamber therebetween. The resultant laser beam escapes from the resonant chamber.
Generally, gas systems are preferred for high average power lasers. Gas lasers are conventionally arranged to have gas flow through the resonant cavity or gain region. Gas lasers are classified in accordance with the process by which the gas laser medium achieves the population inversion. Three conventional varieties of gas lasers are chemical lasers, electric discharge lasers and gas dynamic lasers. Chemical lasers achieve the population inversion by direct generation of higher energy vibrational states in the products of a chemical reaction. Electric discharge lasers achieve the population inversion by "pumping" the higher energy vibrational states in the media through the action of an electric current as in the manner set forth above with respect to the ruby laser. Gas dynamic lasers achieve the population inversion by reducing the population level of the lower energy vibrational state of a hot gas in thermal equilibrium through the rapid cooling caused by supersonic aerodynamic expansion.
Generally the resonators themselves are considered stable resonators, that is, resonators which lie in the stable region of the Fox and Li mode chart and have Gaussian normal modes, or with special limiting cases on the boundary of the stability region, such as the planar resonator. Today, however, the possible application of high power lasers are unlimited in the field of communication, manufacturing, construction, medicine, space exploration and defense and as a consequence thereof, research in this area is ever expanding.
As a result thereof unstable optical resonators now appear to be very useful as resonators for moderate to high gain, large mode volume, high power laser devices. The unstable optical resonators are those resonators which fall well into the unstable region of the Fox and Li mode chart. Although these resonators do not have large diffraction losses, the advantages of these resonators are that they have excellent transverse mode discrimination even at large Fresnel numbers, a large, easily controlled, and uniformly filled mode cross section and diffraction output coupling for high power applications. It is still desirable, however, if such an unstable resonator could improve its mode discrimination and beam quality, and cause the diffraction ripples which reduce local flux load on optical elements.